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Mitosis checkpoints — full lecture notes
Source: Cell biology · Lecture 12 · 42-slide deck · Prof. Bio101 · lecture handout
Why checkpoints matter
Deck- Function: checkpoints halt the cell cycle if something is wrong — DNA damage, incomplete replication, or unattached chromosomes.
- Halt mechanism: checkpoints generate STOP signals that block cyclin-CDK activity at key transitions.
- Failure mode 1: skipping G1/S → cell divides with damaged DNA → daughter cells inherit mutations.
- Failure mode 2: skipping spindle assembly → unequal chromosome segregation → aneuploidy.
- Cancer relevance: 60% of human cancers have a defective p53 — the master checkpoint regulator.
Regulatory machinery
Deck- Cyclins: regulatory subunits whose concentration rises and falls with phases — D, E, A, B cyclins control specific transitions.
- CDKs: cyclin-dependent kinases — catalytic subunits, ALWAYS present but only active when bound to cyclin.
- Cyclin-CDK complex: phosphorylates targets to drive the cell cycle forward; activity is opposed by CDK inhibitors (CKIs).
- MPF (M-phase promoting factor): cyclin B + Cdk1 — its activation triggers mitosis.
G1/S checkpoint
Deck- Location: late G1, before commitment to DNA replication — also called the 'restriction point'.
- DNA damage check: p53 activated by ATM kinase → induces p21 (CKI) → blocks cyclin E-CDK2 → no S-phase entry.
- Growth factor check: without mitogen signals, cyclin D never accumulates → Rb remains active → E2F blocked.
- Rb / E2F switch: active (hypophosphorylated) Rb binds E2F; cyclin D-CDK4/6 phosphorylates Rb → releases E2F → S-phase genes transcribed.
- Cancer relevance: p53, Rb, or p16 loss removes this brake — present in most tumors.
G2/M checkpoint
Deck- What it verifies: DNA replication COMPLETE and no remaining DNA damage before mitosis.
- Sensor proteins: ATM (double-strand breaks) and ATR (single-strand/stalled forks) detect problems.
- Brake mechanism: Chk1/2 kinases inactivate Cdc25 → cannot dephosphorylate Cdk1 → MPF stays off.
- Trigger to enter M: Cdc25 dephosphorylates Cdk1 (cyclin B partner) → MPF active → mitosis begins.
Spindle assembly checkpoint (SAC)
Deck- When: metaphase — anaphase will NOT proceed until ALL chromosomes are bi-oriented on the spindle.
- Sensors: MAD1, MAD2, BUB1, BUB3, BUBR1 at unattached kinetochores produce a 'wait' signal.
- Target: MAD2 sequesters Cdc20 → prevents APC/C activation → securin survives → cohesin stays intact → no separation.
- Anaphase trigger: once all kinetochores are correctly attached, the wait signal vanishes → APC/C-Cdc20 activates → securin destroyed → separase cleaves cohesin → sisters split.
p53 — the master regulator
Deck- Roles: DNA damage sensor, transcription factor, master gatekeeper of all checkpoint responses.
- Targets: induces p21 (CKI), GADD45 (DNA repair), Bax (apoptosis), MDM2 (its own negative feedback).
- Decision: low damage → repair; high damage → apoptosis. p53 levels determine the choice.
- Tumor frequency: TP53 is mutated in ~50% of all cancers — the most-mutated gene in human cancer.
Failure consequences
Deck- Aneuploidy: wrong chromosome count → Down syndrome (trisomy 21), many cancers.
- Apoptosis: controlled cell death — checkpoint's last-resort response when repair fails.
- Genomic instability: checkpoint loss → accumulating mutations → multi-step carcinogenesis.
- Drug response: tumors with intact p53 respond better to chemotherapy; p53-null tumors are drug-resistant.
Cancer connection (exam focus)
Deck- Li-Fraumeni syndrome: germline TP53 mutation → 80%+ cancer risk by age 70.
- Cervical cancer: HPV E6 protein degrades p53 → checkpoint loss → driver of cervical carcinoma.
- Taxol mechanism: stabilizes microtubules → permanent SAC activation → apoptosis. Targets the SAC directly.
- Midterm tip: Prof. flagged: 'expect one cancer-related checkpoint question on the exam.'
